The present invention is directed to inductively coupled plasma-mass spectrometry (ICP-MS), and more particularly to an improved plasma sampling interface for an ICP-MS apparatus for sampling ions from an inductively coupled plasma and introducing the sampled ions to a mass spectrometer.
ICP-MS apparatuses are known in the art. In such apparatuses, an inductively coupled plasma (ICP) is produced by an ICP torch, a sample is introduced into the plasma where the sample is vaporized and ionized, and ions from the sample are sampled from the plasma by a plasma sampling interface and introduced to a mass spectrometer. The plasma sampling interface typically includes a sampler and a skimmer.
In a conventional ICP-MS apparatus, the sampler and the skimmer are usually grounded. U.S. Pat. No. 4,501,965 discloses an ICP-MS apparatus including a quadrupole mass spectrometer which appears to be such a conventional ICP-MS apparatus in that both the sampler and the skimmer appear to be grounded. However, a conventional ICP-MS apparatus can also be operated such that one or both of the sampler and the skimmer are not grounded.
For example, Japanese Patent Application Laid-Open No. 60-133648 discloses an ICP-MS apparatus including what appears to be a quadrupole mass spectrometer in which the sampler is grounded and a DC bias voltage is applied to the skimmer. Absent an English translation of this reference, it is unclear what the magnitude of the DC bias voltage is.
Japanese Patent Application Laid-Open No. 62-64043 discloses an ICP-MS apparatus in which (1) both the sampler and the skimmer are grounded; (2) the sampler is grounded and the skimmer is allowed to float; or (3) the sampler is grounded and the skimmer is connected to ground through a capacitor.
U.S. Pat. No. 4,682,026 discloses an ICP-MS apparatus including a quadrupole mass spectrometer in which (1) an RF bias voltage is applied to both the sampler and the skimmer; (2) the sampler is allowed to float and an RF bias voltage is applied to the skimmer; (3) the sampler is grounded and an RF bias voltage is applied to the skimmer; or (4) the sampler is allowed to float and the skimmer is grounded. The RF bias voltage is in the range of 1 to 10 volts peak-to-peak.
D. J. Douglas, "Some Current Perspectives on ICP-MS", Canadian Journal of Spectroscopy, Vol. 34, No. 2, 1989, pp. 38-49, discusses various ICP-MS apparatuses in which (1) both the sampler and the skimmer are grounded; (2) an RF bias voltage is applied to both the sampler and the skimmer; or (3) the sampler is grounded and an RF bias voltage is applied to the skimmer. The RF bias voltage is in the range of 1 to 10 volts peak-to-peak.
M. Morita et al., "High Resolution Mass Spectrometry with Inductively Coupled Argon Plasma Ionization Source", Analytical Sciences, October 1989, Vol. 5, pp. 609-610, discloses an ICP-MS apparatus with a sector-type mass spectrometer in which a bias voltage up to +5 kV is applied to the sampler. Although this reference does not specify what is done with the skimmer, it is likely that the skimmer is biased at the same voltage as the sampler.
N. Bradshaw et al., "Inductively Coupled Plasma as an Ion Source for High-resolution Mass Spectrometry", Journal of Analytical Atomic Spectrometry, December 1989, Vol. 4, pp. 801-803, discloses an ICP-MS apparatus with a double-focusing magnetic-sector mass spectrometer in which a bias voltage on the order of 4 kV is applied to both the sampler and the skimmer.
PCT International Application Publication No. WO 89/12313 discloses an ICP-MS apparatus with a double-focusing mass spectrometer in which a bias voltage in the range of +4 to +8 kV is preferably applied to both the sampler and the skimmer. A commercial device produced by the assignee of the PCT application operates in this fashion. This reference also discusses known ICP-MS apparatuses in which (1) the sampler is grounded or (2) the skimmer is allowed to float, but does not specify what is done with the skimmer in example (1) or with the sampler in example (2).
U.S. Pat. No. 4,948,962 discloses an ICP-MS apparatus with what appears to be a quadrupole mass spectrometer in which (1) the skimmer is set at a lower potential than the potential at the sampler; or (2) a potential difference is set between the sampler and the skimmer. This reference does not specify whether the sampler or the skimmer is grounded or allowed to float.
U.S. Pat. No. 4,963,735 discloses an ICP-MS apparatus in which (1) both the sampler and the skimmer are grounded; or (2) a DC bias voltage in the range from less than 100 V to more than 200 V is applied to both the sampler and the skimmer.
As known in the prior art, an ICP-MS apparatus can be used for chemical analysis. Alternatively, as disclosed in the applicants' copending U.S. patent application Ser. No. 07/888,613 filed concurrently with the present application and entitled "Ion Processing Apparatus Including Plasma Ion Source and Mass Spectrometer for Ion Deposition, Ion Implantation, or Isotope Separation", an ICP-MS apparatus can be used for ion processing such as ion deposition, ion implantation, or isotope separation.
In chemical analysis, it is desirable that the sensitivity of analysis performed by the ICP-MS apparatus be as high as possible. In ion processing such as ion deposition, ion implantation, or isotope separation, it is desirable that an ion beam produced by the ICP-MS apparatus be as intense as possible. Both the sensitivity of analysis and the intensity of the ion beam can be increased by increasing the ion transmission through the ICP-MS apparatus.